This invention relates generally to implantable medical stimulators, and more particularly to implantable cardioverters and defibrillators.
It has been known for some time that delivery of cardioversion and defibrillation pulses should be synchronized to the patient's own cardiac rhythm, if possible. For example, early synchronized cardioverters and defibrillators are disclosed in U.S. Pat. No. 3,236,239, U.S. Pat. No. 3,527,228, U.S. Pat. No. 3,566,876 and U.S. Pat. No. 4,572,191. All of these references disclose manually triggered synchronized cardioverters. Since that time, the art has progressed substantially, and includes numerous examples of automatically triggered, synchronized cardioverters. Such devices are disclosed in U.S. Pat. No. 3,738,370 issued to Charms et al. and in U.S. Pat. No. 4,384,585, issued to Zipes. Another example of such a deice is the Medtronic Model 7210 implantable cardioverter, which also automatically delivered synchronized cardioversion energy directly to the heart.
In implantable cardioverters, cardioversion and defibrillation pulses are generally delivered from capacitors, which are charged in response to the sensing of a tachyarrhythmia. In the device disclosed in Reissue U.S. Pat. No. 27,757, issued to Mirowski, the heart rhythm is monitored during charging of the output capacitors, and delivery of a cardioversion or defibrillation shock is aborted in the event that the heart returns to a normal sinus rhythm, during capacitor charging. However, most modern implantable cardioverters do not have the capability of sensing the heart's activity during charging of the capacitors. Some such devices, such as the Model 7210 impantable cardioverter discussed above, define a synchronization interval following capacitor charging and abort delivery of the cardioversion pulse if no R-wave is sensed during this interval. An additional approach is embodied in the Ventritex Cadance.TM. defibrillator, which monitors the heart's rhythm during charging of the capacitors, aborting the charging sequence and the delivery of defibrillation or cardioversion pulses in the event that continued presence of tachyarrhythmia is not verified. The Ventritex device also apparently checks the cardiac rhythm after charging, as a prerequisite to delivery of synchronized cardioversion pulses, and aborts delivery of cardioversion pulses in the event that continuing presence of the tachyarrhythmia cannot be confirmed.